Halophila ovalis (R. Br.) Hook. f. from a submarine hot spring in southern Japan

Colonies of the seagrass Halophila ovalis are found growing adjacent to coral Acropora sp. and Seriatopora hystrix in a submarine hot spring (at 15.7 m depth, 28.6°C) at the north coast of Taketomi Island, near the southern tip of Japan. Halophila plants grow in sea water containing sulphide 930 μg S ml −1 and on the substratum with fine precipitates of the submarine hot spring which have sulphide content up to 5400 μg S g −1 DW. The accumulated sulphide concentration reaches as high as 8400 μg S g −1 DW in under ground tissues and 5700 μg S g −1 DW in above-ground tissues, respectively. It is suggested that, not the sulphide concentration but light and possibly water temperature are the limiting factors for the Halophila colonization in the submarine hot spring.